This 1991 annual report presents the results of radioactivity measurements in biosphere samples taken in the Netherlands. The National Measurement Programme (NMP), considered essential for an adequate assessment of radioactivity in the biospere, includes the monitoring of air, deposition and surface water. Milk and grass samples from the surroundings of nuclear reactors are also analysed, just as an overall country milk sample from four milk factories in the Netherlands. Furthermore, this report describes the operationality and analytical data of the National Radioactivity Monitoring Network (LMR) in 1991.